While organic EL devices have been known for about three decades, their performance limitations, mainly short operational life-time, have represented a barrier to many desirable applications. (For brevity, EL, the common acronym for electroluminescent, is sometimes substituted.)
Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, "Double Injection Electroluminescence in Anthracene", RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic emitting material was formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terpheyls, quarterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents. The organic emitting material was present as a single layer having thicknesses above 1 mm.
The most recent discoveries in the art of organic EL device construction have resulted from EL device constructions with the organic luminescent medium consisting of two extremely thin layers (&lt;1.0 micrometer in combined thickness) separating the anode and cathode, one specifically chosen to inject and transport holes and the other specifically chosen to inject and transport electrons and also acting as the organic luminescent zone of the device. The extremely thin organic luminescent medium offers reduced resistance, permitting higher current densities for a given level of electrical biasing. Since light emission is directly related to current density through the organic luminescent medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels (e.g. brightness levels capable of being visually detected in ambient light) to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
For example, Tang U.S. Pat. No. 4,356,429 discloses an EL device formed of an organic luminescent medium consisting of a hole injecting and transporting layer containing a porphyrinic compound and an electron injecting and transporting layer also acting as the luminescent zone of the device.
A further improvement in such organic EL devices is taught by VanSlyke et al U.S. Pat. No. 4,539,507. VanSlyke et al realized a dramatic improvement in light emission by substituting for the hole injecting and transporting porphyrinic compound of Tang an aromatic tertiary amine layer.
The organic EL devices have been constructed of a variety of cathode materials. Early investigations employed alkali metals, since these are the lowest work function metals. Tang et al, U.S. Pat. No. 4,885,211 discloses an EL device requiring a low voltage to operate and which is comprised of a cathode formed of a plurality of metals other than alkali metals, at least one of which has a work function of less than 4 eV.
Commonly assigned VanSlyke et al U.S. Pat. No. 4,720,432 described an EL device using an improved multi-layer organic medium.
Further improvement in organic EL devices such as color, stability, efficiency and fabrication methods have been disclosed in U.S. Pat. Nos. 5,151,629; 5,150,006; 5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687; 4,950,950; 4,769,292, 5,104,740; 5,227,252; 5,256,945; 5,069,975, and 5,122,711.
Specifically, Tang et al in U.S. Pat. No. 4,769,292 discloses that the EL device efficiency can be greatly improved by using an organic emitter layer which comprises of a host material and a small amount of dopant molecules. A preferred host material is an aluminum complex of 8-hydroxyquinoline, namely tris(8-hydroxyquinolinol) aluminum also commonly known as Alq. The dopant molecule is chosen from several classes of highly fluorescent molecules. Perferred examples are the coumarins, rhodamines. Following the teaching of Tang et al, Murayama et al discloses in U.S. Pat. No. 5,227,252 another class of molecules useful as dopant in the organic EL device, namely the quinacridone pigment of formula (I): ##STR2## where R.sub.1 and R.sub.2 are independently hydrogen, methyl group or chlorine or the dehydro form the the quinacridone compound. When used with Alq as the host material, highly efficient organic EL device emitting in the green has been disclosed. However, the usefulness of quinacridone as described in formula (I) is limited because of the instability of the quinacridone molecule in EL operation.